The invention relates to improvements in the performance of low loss optical materials resulting from chemical modification, and to improved polymeric siloxanes.
Organically modified siloxanes (alternating Sixe2x80x94O backboned polymers) have a broad range of applications. In particular, they have good light transmission properties which make them ideal targets for use in optical materials such as optical fibres and devices. They also generally possess good adhesion as well as mechanical and chemical stability over an extended temperature range.
Siloxane polymers can be divided into two broad classes
(i) polysiloxanes prepared by the sol-gel route and
(ii) standard siloxane polymers of the polydiorganosiloxane type.
Polysiloxanes prepared by the sol-gel route are sometimes referred to as ORMOSIL (ORganically MOdified SILicates), ORMOCER (ORganically MOdified CERamics) or inorganic-organic hybrid polymers. These are formed from alkoxysilanes which are normally hydrolysed in the presence of base or acid to yield the corresponding silanol which then undergoes condensation to give a highly cross-linked polysiloxane.
Problematically, these polymers are difficult to process due to their high viscosity. While the condensation processes can be slowed down somewhat to assist in processing, there is always a tendency for such materials to condense so problems due to high viscosity are inevitable.
A further consequence of this unavoidable condensation is the formation of microgels. These microgels make filtration difficult, particularly the passage through 0.2 xcexcm filters, a step which is essential in preparing optical materials to avoid scattering losses.
WO 01/04186 discloses a method for the condensation of diaryl silanediols with trialkoxy silanes. This method produces a polycondensate with the concomitant elimination of alcohol, according to the following scheme:
n Ar2Si(OH)2+n RSi(ORxe2x80x2)3xe2x86x92Polycondensate+2n Rxe2x80x2OH
This synthetic route avoids the presence of large numbers of OH groups which have a high near IR absorption (3500 cmxe2x88x921) that impacts negatively upon optical transparency at 1550 nm. Uncondensed Sixe2x80x94OH groups can also continue a slow reaction over the service life of the polymeric material and lead to cracking and loss of adhesion.
It is desirable to cross-link polymer chains to provide greater chemical stability for the polymer matrix and more importantly to modify the physical properties of the polymer. The most important of these is the ability to cross-link to modify rheology, which in practical terms represents the ability to cure the material from a relatively low viscosity workable polymer to a polymer matrix with sufficient mechanical rigidity to allow use in applications such as optical devices.
WO 01/04186 discloses a number of cross-linking groups such as epoxy and acrylate groups which are pendant from the trialkoxy silane, RSi(ORxe2x80x2)3. There was little or no attention paid to groups which might advantageously provide controlled cross-linking based around the silane diol moiety.
The trialkoxy silane RSi(ORxe2x80x2)3 component is typically used for introducing functionality into the polymer, with the diaryl silane diol having two reactive OH groups and two xe2x80x9cblockingxe2x80x9d aryl moieties.
The approach disclosed in WO 01/04186 means that, for an alternating polymer, 50% of monomer unitsxe2x80x94the trialkoxy silane unitsxe2x80x94have to bear all the desired functionalities, for example, cross-linking, refractive index tuning and fluorination for lower optical loss. This approach is limiting in terms of the synthetic approaches which can be pursued.
It is an object of the present invention to provide polycondensates and polymeric matrices based on the above synthetic route, but which are more readily controlled in terms of structure and functionality.
According to a first aspect the invention provides a storage stable, UV curable, NIR transparent, polycondensate produced by condensation of one or more silanediols of formula (I) and/or derived precondensates thereof 
with one or more silanes of formula (II) and/or derived precondensates thereof 
wherein Ar1 and Ar2 are independently a group with 5 to 20 carbon atoms and at least one aromatic or heteroaromatic group and at least one of Ar1 and Ar2 bears a cross-linkable functional group; and
R1, R2, R3 and R4 are independently alkyl, aralkyl or aryl with up to 20 carbon atoms.
Preferably in the present invention the ratio of formula (I) and formula (II) is 1:1.
The invention also provides a polycondensate of the structure 
wherein
Ar1 and Ar2 are independently a group with 5 to 20 carbon atoms and at least one aromatic or heteroaromatic group and at least one of Ar1 and Ar2 bears a cross-linkable group;
R1 and R2 are independently alkyl, aralkyl or aryl with up to 20 carbon atoms; and
q is at least 1.
According to a second aspect the invention provides a method of production of a polycondensate including the step of condensing one or more silanediols of formula (I) and/or a derived precondensates thereof 
with one or more silanes of formula (II) and/or derived precondensates thereof 
wherein Ar1 and Ar2 are independently a group with 5 to 20 carbon atoms and at least one aromatic or heteroaromatic group and at least one of Ar1 and Ar2 bears a cross-linkable functional group; and
R1, R2, R3 and R4 are independently alkyl, aralkyl or aryl with up to 20 carbon atoms.
Preferably the molar ratio of formula (I): formula(II) is 1:1.
The invention also provides a method of preparing a polycondensate of the structure 
including the step of condensing one or more silanediols of formula (I) and/or derived precondensates thereof 
with one or more silanes of formula (II) and/or derived precondensates thereof 
wherein Ar1 and Ar2 are independently a group with 5 to 20 carbon atoms and at least one aromatic or heteroaromatic group and at least one of Ar1 and Ar2 bears a cross-linkable functional group;
R1, R2, R3 and R4 are independently alkyl, aralkyl or aryl with up to 20 carbon atoms; and
q is at least 1.
According to a third aspect the invention provides a monomer of formula (I) 
when used for the preparation of a storage stable, UV curable, NIR transparent, polycondensate
wherein Ar1 and Ar2 are independently a group with 5 to 20 carbon atoms and at least one aromatic or heteroaromatic group and at least one of Ar1 and Ar2 bears a cross-linkable functional group.
According to a fourth aspect the invention provides a cured polycondensate prepared by curing a polycondensate according to the first aspect, or curing a polycondensate produced according to the second aspect, or curing a polycondensate which includes at least one monomer of the third aspect.
In the above compounds and methods, it is preferable if at least one of Ar1 and Ar2 is a moiety of the type 
In alternative embodiments, at least one of Ar1 and Ar2 is substituted with an epoxy group or a double bond, for instance, an acrylate.
In alternative preferred embodiments, at least one of Ar1 and Ar2 is a moiety of the type 
where L is a connecting group which is selected from alkyl, aralkyl, or ether;
n is 0-5; and
R1, R2, R3 and R4 are independently alkyl, aralkyl or aryl with up to 20 carbon atoms.
Preferably L is selected from the group consisting of: xe2x80x94CH2xe2x80x94, xe2x80x94(OCH2)xe2x80x94 and xe2x80x94(OCH2CH2)xe2x80x94.
Preferably at least one of Ar1, Ar2, R1, R2, R3 and R4 bears at least one fluorine as a substituent.
In alternative embodiments, at least one of Ar1, Ar2, R1, R2, R3 and R4 additionally bears at least one substituent selected from the group consisting of xe2x80x94OH, xe2x80x94SH and xe2x80x94NH2.
In preferred embodiments, at least one of Ar1 and Ar2 is selected from the group consisting of: 
In one highly preferred embodiment, Ar1 is phenyl and Ar2 is 4-styryl. Preferably R1 is selected from the group consisting of CF3(CH2)2xe2x80x94, CF3(CF2)5(CH2)2xe2x80x94, CH3(CH2)2xe2x80x94, xe2x80x94CH3 and phenyl.
Preferably R2, R3 and R4 are independently selected from the group consisting of methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl.
In alternative preferred embodiments, up to 90 mol % of the silane of formula (II) is replaced with a co-condensable compound of boron, aluminum, silicon, germanium, titanium or zirconium.
In another alternative embodiment, up to 90 mol % of formula (I) is substituted with a non-cross-linkable compound, for example diphenyl silane diol.
It is preferred if the polycondensates according the present invention are capable of being photo-structured in layers up to 150 xcexcm in thickness.
In another aspect, the present invention provides a cured condensate and a method of preparing a cured polycondensate including the step of treating a polycondensate of the present invention with a curing agent.
In highly preferred embodiments the curing agent is light. A photoinitiator may be added.
Preferably, the light is UV light and the photoinitiator is selected from the group consisting of: 1-hydroxycyclohexylphenyl ketone, benzophenone, 2-chlorothioxanthone, 2-methylthioxanthone, 2-iso-propylthioxanthone, benzoin, 4,4xe2x80x2-dimethoxybenzoin and mixtures thereof.
In an alternative preferred embodiment, the light is visible light and the photoinitiator may be, for example, camphorquinone.
In further alternative embodiments, other initiators may be added. These may be for example dibenzoyl peroxide, t-butyl perbenzoate and azobisisobutyronitrile.
Furthermore the resin can also be thermally cured using no initiator whatsoever.
The curing temperature is between 80-250xc2x0 C. and more preferably between 170-210xc2x0 C.